The present invention relates to the combining of foreground and background scenes for motion picture or television purposes and particularly relates to the so-called "blue-screen" or special background color screen systems.
It has of course been previously proposed to combine foreground and background scenes by electronic techniques. In this connection, reference is made to my prior U.S. Pat. No. 3,595,987 granted July 27, 1971 and entitled "Electronic Composite Photography", and also to my copending U.S. patent application No. 616,685, filed Sept. 25, 1975, and entitled, "Electronic Composite Photography With Color Control", now U.S. Pat. No. 4,007,487, granted Feb. 8, 1977.
In all of the foregoing systems the principle employed to combine the foreground and background scenes involves the use of a so-called "blue-screen", or other single color background. The blue screen is employed as a backdrop for the foregoing scene, and is very nearly pure blue in color. Other objects having the same color may also be included in the set. Electronic circuitry is provided for detecting the pure signal and for blocking out or turning off the background video when the foreground scene blocks the blue screen. Conversely, when the foreground signal comes from the blue screen or other objects painted the same color, the blue foreground color from the screen or other pure blue objects must be blocked and the background video signals are raised to full intensity.
My U.S. Pat. No. 3,595,987 dealt with two problems which arise in systems of the type generally discussed above. More specifically, when foreground objects include a blue which approaches the blue of the background screen, but has greater green content, such as blue eyes, arrangements must be made to properly reproduce the blue eyes. For normal foreground scenes it had been found sufficient to limit the value of blue to the level of the green present in the sensed signal. To accommodate blue eyes, blue jeans, or similar light blue colors, it was disclosed in my U.S. Pat. No. 3,595,987 to amplify the green signal by which a comparison was made, and limit the blue to "KG", thus permitting the blue to rise to some multiple such as 1.5 times the level of green present in the foreground signal.
U.S. Pat. No. 3,595,987 also disclosed arrangements for accommodating transparent foreground objects such as smoke, glass, or a rapidly moving hand or arm of a foreground character, which appears blurred and therefore partially transparent. This transparency requirement was handled by controlling the level of background video signals in accordance with the magnitude of the difference between blue and green (or amplified green signal), present in the foreground signal. With a pure blue signal representing the background screen, of course, all of the background video would be gated through. However, as the green content increased the level of the background video was dropped.
My U.S. Pat. application Ser. No. 616,685, now U.S. Pat. No. 4,007,487, discloses certain improvements in electronic color systems of this type. Specifically it discloses the use of special circuitry for removing the red and green impurities of the supposedly pure blue background screen. This is accomplished by the provision of a circuit which is sensitive to the transparency of foreground objects, and applying a correction to the foreground red and green signals proportional to this transparency, thereby reducing the red and green signals from the foreground video proportionally to the amount of signal being derived from the supposedly pure blue background screen, and thus removing the red and green impurities arising from the blue screen.
In accordance with another circuit disclosed in my above-cited U.S. Pat. No. 4,007,487, the blue limiting was subject to two conditions. These two conditions serve to both permit proper reproduction of light blue colors in the foreground, and also to suppress "blue flare", or the coloring, of white or flesh tones by the blue light from the blue screen. In accordance with a first condition the blue is limited to green or to the amplified green signal, which may be designated "KG" as mentioned above; and is also at the same time limited to permitting an excess of the blue signal over the green signal only by the difference between the green and red signals. These double criteria arise from the fact that, in blue eyes or blue jeans, the amount of blue, green, and red are a linear function to a substantial extent, with blue exceeding green and green exceeding red by substantially the same amounts. Accordingly, by permitting an increase of blue over the level of green by the difference between the green and red signals, faithful reproduction of foreground light blue signals, such as denims and blue eyes, may be achieved. On the other hand, blue flare from the blue screen on white or flesh tones includes no excess of green over red, and the limitation of the blue to the level of green or KG is assured, thus eliminating this blue flare problem.
While the foregoing systems were adequate for many purposes, they still had some shortcomings. For example, it was not practical with such systems to handle both light blue objects and also purple or magenta in a single scene. Thus, it would not be practical to include in a cast a blue-eyed queen with purple robes. Also, the control of the background video in the foregoing systems was more limited than would be desired. Specifically, it was not capable of providing the varying degrees of dramatic shadow effects which producers and directors frequently desire, nor was the background video sufficiently flexible or correlated with the control of the foreground blue signal.
Another object of the invention is to provide more switching flexibility in the handling of foreground and background signals of the "blue screen" type.
A principal object of the present invention is to overcome these shortcomings of prior systems, and to provide a more comprehensive and flexible system for the control of both blue foreground signals and the level of background video.